The American College of Radiology Imaging Network (ACRIN) is a cooperative group dedicated to conducting clinical trials of diagnostic imaging and image-guided therapeutic technologies as they relate to cancer. The overriding goal of ACRIN is to develop information that results in the earlier diagnosis of cancer, allays the concerns of those who do not have cancer, and improves the length and quality of lives of cancer patients. During its initial funding period, ACRIN developed its infrastructure, developed collaborations, and initiated 18 clinical trials. For the proposed funding period, ACRIN intends to conduct trials that address four key hypotheses: i. PET, MRI, and perhaps other functional and metabolic imaging technologies can predict patient response to treatment earlier than conventional anatomic imaging or observing clinical course. ii. Image-guided interventional techniques can locally control the morbidity of cancer and perhaps reduce the mortality. iii. Screening for cancer using imaging technologies can reduce morbidity and mortality. iv. Molecular imaging of metabolic and physiologic processes can improve cancer diagnosis and treatment. In doing so, ACRIN will perform trials of both emerging technologies that could improve cancer care and established technologies where there is the possibility that the technology is being employed inappropriately. ACRIN trials will focus on the cancers which affect the greatest number of individuals, that produce the greatest morbidity and mortality, and that impose the largest financial costs to society.